plat/arm/sgi: add L0 gpt region to mmap for BL31

R-EL2 would request EL3 for change in GPT mapping when a new realm is
launched. So map L0GPT in EL3 MMU to allow EL3 to update the L0GPT.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>